Method for preparing cooked cereal dough using twin screw preconditioning extruder

ABSTRACT

A dry raw cereal material having a starch fraction is fed into the inlet (38) of a twin screw preconditioning unit (10). The dry material is rapidly advanced and mixed in the first two zones (31, 32) and conveyed into a third zone (33). Water is introduced by ducts (42) and is admixed with the dry material to form a well mixed wetted cereal material. The screws (14, 16) include blank segments (14c, 16c) having radially extending pins (40) to create a material plug in the barrel (30) of the preconditioning unit (10). Steam is added to the wetted material in the next zone (34) to form a heated wetted cereal material which is worked in the fourth zone (34) to form a heated precooked non-continuous cereal compacted dough material which is generally maintained below its gelatinization point. The dough material exits the outlet (41) of the preconditioning unit (10) and enters a low shear extended time cereal cooker (46) which finish cooks the dough material to a fully cooked cereal dough which is formed into desirably shaped and sized pellets.

FIELD OF THE INVENTION

The present invention relates to improved food processing apparatus forpreparing cooked cereal doughs such as for Ready-To-Eat cereals and toimproved methods for preparing cooked cereal doughs.

BACKGROUND

A wide variety of apparatus and methods are known in the art for theprovision of cooked cereal doughs, especially for ready-to-eat (RTE)cereal products. Such apparatus and methods convert a raw dryungelatinized cereal material and water into a cooked cereal doughcharacterized by hydration and gelatinization of the starchyconstituents of the cereal material. Important aspects of cereal cookinginclude not only the degree of gelatinization but also the texture ofthe cooked cereal dough.

A well known cereal cooking apparatus is known in the art as a JamesCooker. The James Cooker provides a low shear, low pressure, extendedtime (e.g., 30 to 180 minutes) type of cooking yielding a cooked cerealdough that has a highly developed cooked flavor but has not experiencedhigh amounts of shear. The dough is extruded under low shear through dieplates with die holes to produce sized and shaped cooked cereal doughpellets. The basic design and operation of the James Cooker is describedin U.S. Pat. Nos. 2,233,919 (issued Mar. 4, 1941 to T. R. James),2,263,301 (issued Nov. 18, 1941 to T. R. James), and 2,272,007 (issuedFeb. 3, 1942 to T. R. James), each of which is incorporated herein byreference. Over the years, various improvements have been made to theJames Cooker (see, for example, U.S. Pat. No. 5,433,490, issued Jul. 18,1995 to R. Hurd and S. Liedman which describes a quick change diemechanism especially useful for the James Cooker for the rapid changeouts of plugged dies, also incorporated by reference herein).

The present invention provides further improvements in the James Cooker,namely; reductions in downtime, improvements in throughputs, andimprovements in cook consistency and degree of control. The presentinvention involves mounting a twin screw preconditioning unit onto theinlet of the James Cooker for mixing, hydrating, heating and convertinga cereal feed material into precooked crumbly dough material prior tofeeding into the James Cooker.

James Cookers are typically run 24 hours a day in product runs fromseven up to 21 or more days. During such extended production runs,however, the cookers frequently require being brought temporarilyoff-line due to plugging of the dies. Fouling or even plugging of thedies is most frequently caused by dry, hard dough balls in which thecereal material is incompletely cooked due to incomplete hydration ofthe cereal material. Changing out the die can require several hoursbefore a clean die is in place and the cooker is brought up to steadystate conditions. Such die change outs are required at unpredictableintervals and may happen several times a day. Also, a great quantity offood material that is inconsistently processed before the cookers reachsteady state conditions must be discarded. In the '490 patent, thisproblem was addressed by an improvement in the outlet end of the JamesCooker, namely, by providing a faster means for changing out the pluggeddies.

However, the present invention provides an improvement in the inlet endof the James Cooker. Specifically, the present invention resides in partin adding a particular twin screw preconditioning unit. By adding thepresent twin screw preconditioning unit, improvements are obtained inthe consistency of hydration of the cereal feed material. Such hydrationconsistency improvements lead to reductions in the incidence of dieplugging from dry dough balls. Reductions in downtime alone andattendant material waste represent significant cost savings, especiallyover extended production runs. The present invention can be used aloneor in combination with the quick die changer improvements described inthe '490 patent.

Such further reductions in downtime due to fewer plugged dies providedby the twin screw preconditioning unit alone can result in productivityimprovements of 10 to 20%.

In addition to reducing downtime, employment of a twin screwpreconditioning unit increases significantly the throughput of suchJames Cookers. Conceptually, the James Cooker has three zones in itscooking section that 1) mixes the ingredients, 2) hydrates the mixture,and 3) cooks the hydrated mixture. In addition to its cooking section,the James Cooker also includes a working section that works the cookedcereal material into a dough and extrudes the dough through the dieplate to form cooked cereal dough pellets. The twin screwpreconditioning unit performs the functions previously performed in thefirst two zones of cooking section of the James Cooker allowing for amore rapid and thus higher throughput of cereal material. Theimprovements in throughput can range from 10 to 25%.

By both reducing downtime and by increasing throughput, the totalincrease in output can be from about 20 to 45% or even more. In view ofthe expense of such James Cookers, such increases in output lead todramatic cost savings and increased productivity.

Of course, rotating paddle types of cereal preconditioners are known foruse immediately upstream of high shear extruders (see for example, U.S.Pat. No. 5,120,559, issued Jun. 9, 1992 to S. H. Rizvi, U.S. Pat. No.4,285,271, issued Aug. 25, 1981 to Falck et al; and/or U.S. Pat. No.4,665,810, issued May 19, 1987 to Falck). However, such preconditionersgenerally involve the mere passive exposure to wet moist steam toaccomplish hydration. Also, the output material from suchpreconditioners must necessarily be free flowing, granular and fluffy inorder to feed the material to single or twin screw extruders, incontrast to the more dense compacted precooked cereal dough materialthat exits the twin screw preconditioning unit of the present invention.

Still another advantage of the present invention resides in the abilityto control the consistency of the cooked dough with respect to bothtexture and flavor. Moreover, conventional cereal preconditioners, andeven prior usage of the James Cooker, typically require close control ofthe particle size of the cereal material. Since such preconditioners andthe James Cooker passively expose the material to wet steam, closeattention to the surface area-to-volume ratio must be made to ensureproper hydration. In contrast, using the present twin screwpreconditioning unit allows for employment of a wide variety of particlesize feed materials without materially adversely affecting the abilityto control the desired cooked dough's properties.

Similarly, the consistency of hydration of the cereal mixture by usingthe twin screw preconditioning unit also results in a more consistentcook in the James Cooker in addition to the reduction in the incidenceof die plugging. A more consistent cook results in a gain in productquality and may result in enhanced flavor development in the dough whichwas not previously possible with the James Cooker alone. The degree ofhydration obtained independent of particle size, the more consistentcook, and the other attributes resulting in the utilization of the twinscrew preconditioning unit with the James Cooker reduce the variabilityof the operating parameters and lend to automatic control to furthersimplify operator interface in the preparation of cooked cereal doughs.

Improved methods for preparing cooked cereal doughs having specialapplication in the production of RTE cereals according to the preferredteachings of the present invention will become clearer in light of thefollowing detailed description of an illustrative embodiment of thisinvention described in connection with the drawings.

DESCRIPTION OF THE DRAWINGS

The illustrative embodiment may best be described by reference to theaccompanying drawings where:

FIG. 1 shows a side elevational view of a twin screw preconditioningunit utilized in the preparation of cooked cereal doughs according tothe preferred teachings of the present invention.

FIG. 2 shows a top plan view of the twin screw preconditioning unit ofFIG. 1.

FIG. 3 shows a cross sectional view of the twin screw preconditioningunit of FIG. 1 according to section line 3--3 of FIG. 1.

All figures are drawn for ease of explanation of the basic teachings ofthe present invention only; the extensions of the figures with respectto number, position, relationship, and dimensions of the parts to formthe preferred embodiment will be explained or will be within the skillof the art after the following description has been read and understood.Further, the exact dimensions and dimensional proportions to conform tospecific force, weight, strength, and similar requirements will likewisebe within the skill of the art after the following description has beenread and understood.

Where used in the various figures of the drawings, the same numeralsdesignate the same or similar parts. Furthermore, when the terms"first", "second", "length", "end", "axial", "radial", "longitudinal","upstream", "downstream", and similar terms are used herein, it shouldbe understood that these terms have reference only to the structureshown in the drawings as it would appear to a person viewing thedrawings and are utilized only to facilitate describing the illustrativeembodiment.

DESCRIPTION

A twin screw preconditioning unit according to the preferred teachingsof the present invention is shown in the drawings and generallydesignated 10. Generally, unit 10 is in the form of an extruder andincludes substantially intermeshing screws 14 and 16 rotatably mountedinside of a housing assembly 18. In the most preferred form, housingassembly 18 is not jacketed. Screws 14 and 16 can include suitable sealsand bearings 20 for rotatably mounting and sealing the shaft ends ofscrews 14 and 16 to housing assembly 18 to generally prevent feedmaterials and precooked dough from leaking along the shafts of screws 14and 16. A suitable drive 21 can be provided such as at the upstream endsof the shafts of screws 14 and 16 for co-rotating screws 14 and 16inside of housing assembly 18 and in the most preferred form is of thevariable speed type.

Assembly 18 includes a barrel or channel 30 of a FIG. 8-shape of a sizeand configuration corresponding to intermeshing screws 14 and 16 andspecifically providing minimal screw-to-barrel clearance when screws 14and 16 are located in channel 30. Assembly 18 can be formed in anydesired manner including utilizing conventional barrel sections. In themost preferred form, assembly 18 is formed by a first housing 22 of agenerally U-shape and by a cover 24 of a generally planar shape which isremovably secured to the upper edges of first housing 22.

For purposes of explanation, unit 10 and specifically screws 14 and 16inside of channel 30 can be considered as including five functionalzones 31-35, with the material moving downstream from zone 31 to zone32, zone 32 to zone 33, etc.

First zone 31 of unit 10 is for feeding the dry raw feed material intobarrel 30 and generally includes an inlet 38 formed in housing assembly18. In the most preferred form, inlet 38 is formed in cover 24 and iscylindrical in shape having circular cross sections. Inlet 38 has adiameter generally equal to the maximum lateral extent between theshafts of screws 14 and 16 and is positioned with its lateral extentgenerally corresponding to the maximum lateral extent between the shaftsof screws 14 and 16. Screws 14 and 16 include flights 14a and 16a withinzone 31 for rapidly advancing dry material entering channel 30 ofhousing assembly 18 from inlet 38 into zone 32 and for very coursemixing of the dry material. In the preferred form, the longitudinalextent of inlet 38 is within the longitudinal extent of flights 14a and16a and in the most preferred form is less than the longitudinal extentof flights 14a and 16a with inlet 38 positioned at the upstream end offlights 14a and 16a.

Second zone 32 of unit 10 is for initial mixing and conveying the dryraw feed material from zone 31 to zone 33. Screws 14 and 16 includeflights 14b and 16b within zone 32, with flights 14b and 16b having aradial extent equal to flights 14a and 16a but being of a greater pitchthan flights 14a and 16a. Zone 32 in the preferred form has alongitudinal length slightly less that zone 31 and in the preferred formis generally two thirds the longitudinal length of zone 31.

Third zone 33 of unit 10 is for creating a material plug in barrel 30and for further mixing the material. Screws 14 and 16 within zone 33each include a blank segment 14c and 16c which is free of conveyingflights. In the most preferred form, blank segments 14c and 16c eachinclude a plurality of pins 40 extending radially from the shafts ofscrews 14 and 16 to a radial extent generally equal to that of flights14a, b and 16a, b. In the most preferred form, three pins 40 arelongitudinally spaced along the shafts every 90° around the shafts ofscrews 14 and 16. Additionally, in the most preferred form, pins 40 ofscrew 14 are positioned longitudinally intermediate pins 40 of screw 16,with the first pins 40 of screw 14 being positioned longitudinallyupstream of the first pins 40 of screw 16, the second and third pins 40of screw 14 being positioned intermediate the first and second and thesecond and third pins 40 of screw 16, and with the third pins 40 ofscrew 16 being positioned longitudinally downstream of the third pins 40of screw 14. Zone 33 in the preferred form has a longitudinal lengthgenerally equal to but slightly less than the longitudinal length ofzone 32.

Fourth zone 34 of unit 10 is for providing residence time forconditioning and precooking the material into a precooked dough materialand for conveying the material from zone 33 to zone 35. Screws 14 and 16include flights 14d and 16d within zone 34, with flights 14d and 16d inthe most preferred form being of the same radial extent and pitch asflights 14b and 16b. In the most preferred form, zone 34 has asubstantial longitudinal length which is slightly greater than one halfof the total longitudinal length of barrel 30 of unit 10.

Fifth zone 35 of unit 10 is for allowing the exit of the conditionedprecooked dough material from barrel 30 and generally includes an outlet41 formed in housing assembly 18. In the most preferred form, outlet 41is rectangular in shape having a lateral width generally equal to thelateral extent of barrel 30 and having a longitudinal width generallyequal to the longitudinal extent of zone 35.

Screws 14 and 16 include flights 14e and 16e within zone 35, withflights 14e and 16e in the most preferred form being of the same radialextent and pitch and being continuous with flights 14d and 16d. Flights14e and 16e have a longitudinal extent generally equal to but slightlyless than one half that of zone 35 and outlet 41.

Screws 14 and 16 further include flights 14f and 16f within zone 35 andwhich are in a reverse direction of flights 14e and 16e and extend fromthe downstream end of barrel 30 towards the upstream end. In the mostpreferred form, flights 14f and 16f have the same radial extent andpitch as flights 14e and 16e (but reversed) and have a longitudinalextent generally equal to but slightly less one half that of zone 35 andoutlet 41. The function of flights 14f and 16f is to generally preventthe precooked dough material from advancing to the downstream end ofhousing assembly 18. Material reaching the downstream end of housingassembly 18 may have a tendency to cause excessive wear in and/or toenter seals and bearings 20 for the downstream ends of screws 14 and 16.

Flights 14e and 14f and flights 16e and 16f are interconnected togetherby axially extending plates 14g and 16g which extend radially outwardfrom the shafts of screws 14 and 16 to a radial extent equal to flights14a, b, d, e, f and 16a, b, d, e, f. In the most preferred form, plates14g and 16g are arranged at the same angular position on screws 14 and16 in barrel 30. It can be appreciated that the pressure of theprecooked dough material entering zone 35 drops as it leaves flights 14eand 16e and prior to its exiting through outlet 41. In the preferredform, the longitudinal length of zone 35 is generally equal to thelongitudinal length of zone 32.

Unit 10 further includes provisions for introducing moisture andsolutions into barrel 30 and the material being conveyed by screws 14and 16. In the most preferred form, housing assembly 18 of unit 10includes a pair of ducts 42 formed adjacent to the downstream end ofzone 32 and another pair of ducts 44 formed adjacent to the upstream endof zone 34. In the most preferred form, ducts 42 and 44 extend at adecreasing acute angle in the order of 45° relative to the axes ofscrews 14 and 16 in the flow or movement direction of the material andin a plane generally parallel to a plane including the axes of bothscrews 14 and 16.

Outlet 41 of twin screw preconditioning unit 10 is directly connected tothe inlet of a conventional James Cooker 46 according to the preferredteachings of the present invention. In particular, James Cooker 46 canbe of the form shown and described in U.S. Pat. Nos. 2,233,919;2,263,301; 2,272,007; and 5,433,490. However, it is believed that unit10 can be utilized and can have special application for other low shear,low pressure, (0 to 100 psig) extended time type of cooking apparatus.In the most preferred form, unit 10 is mounted directly to the frame ofJames Cooker 46 and can extend over, in front of, or to either side ofJames Cooker 46.

Now that the basic construction of unit 10 according to the preferredteachings of the present invention has been set forth, improved methodsfor preparing cooked cereal doughs such as for RTE cereal products canbe explained in the most preferred form utilizing unit 10 of the presentinvention. In particular, drive 21 can be actuated to rotate screws 14and 16 inside of housing assembly 18. Dry raw feed material isintroduced into inlet 38 in any suitable manner, with the feed materialin the preferred form being at ambient temperatures in the order of 50to 120° F. (10 to 50° C.) depending upon season, plant location, storageconditions, etc.

Useful herein for the cereal feed materials are a wide variety of cerealmaterials derived from such common cereal as wheat, barley, oats, corn,triticale or other cereal grains and mixtures thereof. The cereal feedmaterials, of course, can also optionally include conventional cerealingredients such as salt, minerals, malt syrup, sugar(s), fiber (e.g.,bran, cellulose, pectin, psyllium), vitamins, flavor and colorants.

In a preferred embodiment, the cereal feed materials comprise a wholegrain ingredient, e.g., soft wheat or whole oats. In other embodiments,various cereal feed materials such as cereal flours (whether whole grainor a cereal flour fraction) or cut cereal pieces can be used.

The particle size of the cereal feed materials is not critical, and itis an advantage of the present invention that the particle size can varywithout materially adversely affecting the cooked cereal dough productsobtained. Useful herein are flours, grits, flakes and other sizes andshapes of grain or cereal materials.

The dry feed material introduced into inlet 38 falls into and engagesflights 14a and 16a which quickly convey the dry feed material from zone31 to zone 32. The dry feed material is very coarsely mixed while beingconveyed by flights 14a and 16a.

Due to the greater pitch of flights 14b and 16b, the dry feed materialis mixed while being conveyed by flights 14b and 16b and obtains furtherfill in the FIG. 8-shape of barrel 30. While in zone 32, the dry feedmaterial is admixed with sufficient amounts of water introduced intobarrel 30 through ducts 42 in the preferred form at a temperature in theorder of 90 to 120° F. (30 to 50° C.). While in zone 32, the dry feedmaterial and water are mixed to form a well mixed wetted cereal feedmaterial having a moisture content of about 25 to 40%.

Optionally, the present methods can additionally include the step ofadding a sugar solution to the wet mixture to provide a sweetenedmixture having a sucrose content of between about 1 to 5%. Such sugarsolution could be added with the water through ducts 42 or could beadded separately such as through further ducts. A portion of the totalmoisture content is thus provided by the sugar solution.

It should be noted that the provisions of flights 14a, b and 16a, b ofdiffering pitches is advantageous in keeping zone 31 dry and inparticular from keeping water and other solutions introduced into zone32 such as though ducts 42 from entering zone 31 and particularly fromreaching inlet 38. If moisture reaches inlet 38, bridging of inlet 38 bythe feed material can occur which requires operator attention andreduces operation efficiency. Specifically, according to the preferredteachings of the present invention, ducts 42 introduce the water andsolutions adjacent to the downstream end of zone 32 so that the feedmaterial substantially fills barrel 30 to prevent an unobstructedpassage to inlet 38. Additionally, as flights 14a and 16a rapidlyadvance the feed material into zone 32, the feed material is availableto absorb and intermix with the water and solutions which are then notfree to continue to travel upstream towards inlet 38.

Flights 14b and 16b deliver the well mixed wetted feed material to blanksegments 14c and 16c. As blank segments 14c and 16c are free of flights,rotation of screws 14 and 16 does not result in movement of the wettedfeed material in zone 33. Rather movement of the wetted feed material iscaused by the subsequent introduction of wetted feed material into blanksegments 14c and 16c. Thus, the wetted feed material tends to fillbarrel 30 in zone 33 and forms a plug which moves downstream.

Rotation of screws 14 and 16 causes pins 40 to rotate in the moving plugin zone 33. Pins 40 passing through the wetted feed material furthermixes the wetted feed material in zone 33 and specifically within thelongitudinal extent of blank segments 14c and 16c. As the wetted feedmaterial is pushed through zone 33 by the subsequent introduction ofadditional feed material, it enters zone 34 and is engaged by flights14d and 16d. Thus, the rotation of screws 14 and 16 causes flights 14dand 16d to convey the heated well mixed wetted feed material downstream.

Thereafter, steam is added to the well mixed wetted cereal material byits introduction into barrel 30 through ducts 44 in the preferred form.Steam is added in sufficient amounts to form a heated wetter feedmaterial having a temperature of about 180 to 220° F. (82 to 104° C.),preferably about 190to 215° F. (88 to 102° C.) and most preferably about210 to 215° F. (99 to 102° C.). The steam can be any type of steam andconveniently is wet, intermediate pressure (30 to 60 psig, 310 to 515kPa.) steam. The steam, upon condensation, provides about one part in 10of the required moisture.

The steam provides substantially all of the heat for cooking of the wellmixed wetted cereal material into the precooked dough material, withradiant and conductive heat also arising by virtue of the closeproximity to James Cooker 46 and by virtue of mechanical energy.

It should then be appreciated that the material plug formed in blanksegments 14c and 16c functions as a plug for preventing steam introducedthrough ducts 44 from passing upstream and specifically through zone 33and into zones 31 and 32. It can be appreciated that loss of steamthrough inlet 38 is undesirable for several reasons including but notlimited to increased operational costs, difficulties in introducing theraw feed material into inlet 38, and increased safety hazards andconcerns for surrounding personnel and equipment.

Due to the increased temperature provided by the steam, it can beappreciated that the heated wetted feed material will be worked, cookedor otherwise conditioned while it moves through zone 34 due to therotation of flights 14d and 16d. Specifically, the heated wetted feedmaterial will turn into a precooked dough material generally prior toits entry into zone 35.

The precooked dough material, while heated, is importantly below itscomplete gelatinization point and thus does not constitute a fullycooked gelatinized cereal dough. Rather, the dough material ischaracterized by being hydrated, warmed and precooked.

The extent of cooking and various characteristics of a starchy materialare frequently analyzed and described in the art using a Rapid ViscosityAnalyzer ("RVA"). The RVA instrument subjects a sample material admixedwith cold (25° C. ) water, heats the sample during a prescribedtime/temperature regimen and measures the viscosity of thematerial/water sample over time and temperature. The viscosity isexpressed in Rapid Visco Units ("RVU", generally 1 RVU=11.9 centipoise.)over time to provide a pasting curve. The peak value on the curve "peakpasting value" is thus expressed in RVU units. Generally, a raw cerealflour will have a peak pasting value of about >700 RVU indicating nogelatinization. The finished cooked cereal doughs herein arecharacterized by peak pasting values ranging from about 150 to 300 RVUindicating substantially complete gelatinization. The precooked doughmaterial exiting twin screw preconditioning unit 10 is essentiallycharacterized by peak pasting curve values ranging from about 300 to 500RVU.

Another alternate way of expressing the desirable degree of cookingherein is the percentage of starch that is completely gelatinized. Thepresent precooked dough material exiting twin screw preconditioning unit10 is essentially characterized by a gelatinized starch fraction rangingfrom about 20 to 60% and preferably about 30 to 60%.

Upon complete gelatinization, a cooked cereal dough is transformed intoa continuous material having a viscosity that is several orders ofmagnitude greater than the uncooked, ungelatinized dough materialexiting twin screw preconditioning unit 10. When this transition occurs,the torque load on the motor driving the twin screw shafts increasesdramatically. Depending upon the working capacity of the electric motordriving the shafts, the motor can undesirably cut-out completely leadingto a freeze up of the system. Moreover, a fully gelatinized dough isdifficult to feed into the inlet of James Cooker 46 by gravity feed fromthe discharge of twin screw preconditioning unit 10.

The precooked dough material leaving zone 34 enters into zone 35 whereit exits unit 10 through outlet 41. It should be appreciated that due tothe provision of plates 14g and 16g and reversed flights 14f and 16f,the pressure of the precooked dough material exiting outlet 41 isreduced from the pressure of the precooked dough material just beforeleaving zone 34.

The precooked dough material exiting twin screw preconditioning unit 10has a temperature of about 180 to 220° F. (82 to 104° C.), preferablyabout 190 to 215° F. (88 to 102° C.) and most preferably about 210 to215° F. (99 to 102° C.). The precooked dough material has a moisturecontent of about 27 to 40%, preferably about 30 to 38%.

The precooked dough material is compacted and crumbly and has a densitythat ranges from about 70 to 80 lb./ft³. (1.12 to 1.28 g/cc), andpreferably about 1.13 to 1.17 g/cc.

In contrast, precooked cereal material exiting a conventional paddletype preconditioner is lighter in density (60 to 65 lb./ ft³.),different in texture (i.e., is pulverant and free flowing) and is lowerin the percentage of gelatinization (i.e., <15%). These differencesinherently result from the differences in design and operation of apaddle type preconditioner. In a paddle type preconditioner, therotating paddles toss the cereal material upwards thereby exposing thesuspended material to the wet steam environment. If the paddle shaft isturned too slowly, the material is not thrown upwards. If the materialis gelatinized too much, then the cereal material becomes sticky andbegins to stick to the paddles and is not thrown upwards to furtherhydrate and further gelatinize. As a result, the upper limit on thepercentage of gelatinization is less than that obtained in twin screwpreconditioning unit 10 of the present invention.

The residence time within twin screw preconditioning unit 10 accordingto the preferred teachings of the present invention is very brief,ranging on the order of about 10 to 30 seconds, and preferably about 10to 20 seconds. Such a short processing time is in part due to the rapidabsorption of steam by the cereal material by virtue of the steam beingadded after the material is thoroughly wetted. Specifically, theinjection of steam into ducts 44 of unit 10 according to the teachingsof the present invention provides key attributes which dramaticallyimprove the cooking process. Particularly, exceptional and quickingredient hydration occurs within the residence time in unit 10 ascompared to the thirty or more minutes which were required in JamesCooker 46 alone. Also, exceptional and quick preheating of the doughmaterial to saturation temperatures occurs within the residence time inunit 10 as compared to the thirty or more minutes which were required inJames Cooker 46 alone.

The operating pressure within twin screw preconditioning unit 10 of thepreferred form of the present invention is much lower than on aconventional cooking twin screw extruder and ranges from about 1 to 5psig (108 to 136 kPa), and preferably about 1 to 2 psig (115 to 136kPa).

The present precooked cereal dough material exiting twin screwpreconditioning unit 10 has a pourable, or crumbly or discontinuousconsistency. The precooked cereal dough material forms a compacted doughas compared to a continuous dough exiting the James Cooker. Also, thecrumbly precooked dough material is distinguishable from the freeflowing pulverant material that is prepared from a conventionalpreconditioner.

Thereafter, the precooked (or equivalently herein, "par-cooked" or"partially cooked") cereal dough material exiting twin screwpreconditioning unit 10 is fed directly, such as by gravity falling,into the inlet ofJames Cooker 46. The precooked cereal dough material isthen finished cooked for about 30 to 90 minutes, preferably on the orderof about 50 to 70 minutes, at about 220 to 240° F. without shear to forma fully cooked cereal dough.

This represents a greatly improved performance capability of JamesCooker 46 over James Cooker 46 utilized alone which had a totalresidence time preferably on the order of 70 to 90 minutes. Thisimproved performance is due to the complete material premixing withsteam and the precooking obtained by twin screw preconditioning unit 10according to the teachings of the present invention. In particular, thezones of the cooking section of James Cooker 46 previously utilized tomix and hydrate the feed material are freed to be used for cooking asthe precooked dough material leaving outlet 41 of unit 10 of the presentinvention is thoroughly mixed and hydrated. Thus, use of unit 10 of thepresent invention effectively increases the cooking length of JamesCooker 46 by the length previously required to mix and hydrate thematerial, which could be one third of the length of James Cooker 46.Increased cooking length of James Cooker 46 provides increasedflexibility for both product enhancement and equipment operation.

Operation of twin screw preconditioning unit 10 according to thepreferred teachings of the present invention has resulted in therecognition of several advantages in the methods for preparing cookedcereal doughs and in the preferred form of unit 10 of the presentinvention. Specifically, it has been found that the arrangement of ducts42 and 44 according to the preferred teachings of the present inventionis particularly advantageous. First, the decreasing angle of ducts 42and 44 reduces the tendency of material moving in barrel 30 by therotation of screws 14 and 16 from backing into and plugging or otherwiseblocking ducts 42 and 44. Additionally, the preferred positioning ofducts 42 is sufficiently downstream of inlet 38 and zone 31 so that thematerial upstream of duct 42 and within zone 32 generally preventspassage of moisture to zone 31 of unit 10. Likewise, the preferredpositioning of ducts 44 in the upstream end of zone 34 insures that thewetted feed material in blank segments 14c and 16c creates a materialplug for preventing steam introduced through ducts 44 from passing therebeyond while presenting sufficient residence time in zone 34 to arriveat precooked dough material of the desired characteristics.

Still another advantage of the present invention is that increases inoutput are obtained without increasing the footprint of the equipment inan existing facility.

Still another advantage resides in the safety of the combination of twinscrew preconditioning unit 10 with James Cooker 46. Previously, whenparticulate feed material is fed to James Cooker 46, the steam andhumidity can cause the particulate material to block or to form a bridgeacross the inlet of James Cooker 46. Such blocking or bridging requiredthe operator to attempt to break the bridge using a stick. Once broken,the pressure within James Cooker 46 can cause a sputtering of the steamand hot materials. Frequently, such sputtering would cause burning orother operator injury. By directly feeding the output from twin screwpreconditioning unit 10 to James Cooker 46, such bridging issubstantially eliminated, thereby greatly reducing the incidence andseverity of operator injury.

Still another advantage is that by directly connecting the output oftwin screw preconditioning unit 10 to the inlet of James Cooker 46,atmospheric contamination of the cooked material is eliminated.

Still another advantage is that by directly connecting the output oftwin screw preconditioning unit 10 to the inlet of James Cooker 46,spillage loss in charging James Cooker 46 is essentially eliminated.

Still another advantage is that since a significant amount of theprocessing occurs in twin screw preconditioning unit 10, James Cooker 46can be brought up to desired steady state conditions faster than JamesCooker 46 alone, leading to further improvements in productivity andoutput. Similarly, for the same reasons, changeovers and shutdowns canbe performed quickly.

Thus since the invention disclosed herein may be embodied in otherspecific forms without departing from the spirit or generalcharacteristics thereof, some of which forms have been indicated, theembodiments described herein are to be considered in all respectsillustrative and not restrictive. The scope of the invention is to beindicated by the appended claims, rather.than by the foregoingdescription, and all changes which come within the meaning of the claimsare equivalency of the claims and are intended to be embraced therein.

What is claimed is:
 1. A method for preparing a cooked cereal dough,comprising the steps of:feeding a dry raw cereal material having astarch fraction to a twin screw preconditioning extruder; introducingwater into the preconditioning extruder to admix water to the dry rawcereal material to form a well mixed wetted cereal material having amoisture content of about 25 to 40%; thereafter adding sufficientamounts of steam to the wetted cereal material within thepreconditioning extruder to form a heated wetted cereal material havinga temperature of about 180 to 220° F. (82 to 104° C.); working theheated cereal material for 10 to about 30 seconds while maintaining thecereal material below its gelatinization point to form a heatedprecooked non-continuous cereal compacted dough, said dough having: atemperature of about 180 to 220° F. (82 to 104° C.), a moisture contentof about 27 to 40%, a density of about 70 to 80 lb./ft³. (1.12 to 1.28g/cc) and being discontinuous in form; discharging the heated precookednon-continuous cereal compacted dough from the twin screwpreconditioning extruder into a low shear extended time cereal cooker;and cooking the dough for about 30 to 90 minutes without substantialshear to form a low shear cooked cereal dough.
 2. The method of claim 1additionally comprising the step of: forming the cooked cereal doughinto desirably shaped and sized pellets.
 3. The method of claim 2wherein the feed material is at ambient temperatures.
 4. The method ofclaim 3 wherein the precooked dough is about 30 to 60% starchgelatinized.
 5. The method of claim 4 wherein the feeding step comprisesthe step of feeding the dry raw cereal material to the twin screwpreconditioning extruder comprising, in combination: a housing assemblyincluding a channel adapted to receive intermeshing screws; first andsecond intermeshing screws located in the channel and rotatably mountedin the housing assembly; an inlet formed in the housing assembly for theintroduction of the dry raw cereal material into the channel; an outletformed in the housing assembly for exiting of the ungelatinized doughfrom the channel, with the first and second screws including an inletzone for conveying the material from the inlet towards the outlet, anoperative zone for working the material moving between the inlet and theoutlet, and an outlet zone moving the ungelatinized dough through theoutlet and into the low shear extended time cereal cooker; a first ductfor introducing steam into the channel in the operative zone of thescrews; means in the channel upstream of the first duct for creating amaterial plug in the channel for generally preventing steam introducedthrough the first duct from passing upstream there beyond; a second ductfor introducing water into the channel upstream of the material plug;and a mixing zone intermediate the inlet zone and the material plug formixing the material, with the second duct introducing the water into thechannel within the mixing zone, with the water introducing stepcomprises the step of introducing water into the second duct, with theadding step comprises the step of introducing steam into the first duct.6. The method of claim 5 wherein the creating means comprisess blanksegments in the first and second screws and which do not result inmovement of the material as the result of rotation of the first andsecond screws.
 7. The method of claim 6 wherein the twin screw extruderfurther comprises, in combination: a plurality of pins extendingradially from the first and second screws and within the blank segmentsfor mixing the material in the blank segments.
 8. The method of claim 4wherein in the introducing water step, the water contains sufficientamounts of sugar to provide the dough with a sugar content of about 1%to 5%.
 9. The method of claim 4 wherein the operating pressure withinthe twin screw preconditioner ranges from about 1 to 5 psig.